If I recall correctly, I did a search while a video was being played. Then, when I pressed enter on the query, a list of results appeared on top | bottom of the video.

If I selected one of the videos on the search results, they were auto-added on a right playlist. If you wanted to move to the next video, you could select it on the right playlist.

I found it really intelligent and useful, but at the start it felt a little weird.

My assumption was that it was too difficult for everyone, and Youtube decided to keep playlists as they were.

As for in which period of time I remember this, I was in Uppsala from 28 Apr 2010 to 04 May 2010